We’ve locked bootloader for Redmi Note 3 since its launching in order to
protect user data safety. As we said before, we’ll be making the same
change for other Mi devices gradually. Now, the list will include Mi 4c
and Mi Note Pro also.

You would need to achieve a certain status on their forums in order for xiaomi to approve your unlock request.

The most interesting part is that once approved, you would need to wait
for their SMS containing the unlock code for you to actually unlock your
device which may take 15 days or more according to the some of the
users who tried unlocking.